Cannot light portions of object with point light
While im not a Pro. I Think, I THINK. You should try increasing the strength of the Light. Or try using multiple lights. I Could also say Try to Transparent the background and Raise the color of the background to a bright color Preferrably a Bright White This will make the Background transparent, And the objects in the rendered image will be Nice and bright.
If you want to have the background grey i won't reccomend the latter.

One of my renders of a character for one of my games. With one default pointlight just above the head, Not bright, I Would like it more bright.  
Now i have changed the Strength of the Pointlight to 900.
As you may notice the ribs are more bright And so are alot of the other parts.